DC power is needed for many types of telephone communication equipment and for control equipment used at electric utility substations and power plants. The DC power, required for this communication equipment and control equipment used at utility substations and power plants, is normally supplied by a DC power source which, in turn, is supplied with AC power from an AC power source. A typical DC power source for these applications is a battery charger.
A bank of standby batteries is utilized as a backup DC power source when the normally AC powered, DC power source either cannot supply all the power required by the components or when the AC power supply goes off line, as during a power failure at the local electric utility.
Standby storage batteries are designed to deliver energy to a load over a relatively long period of time at a slowly declining voltage, in contrast with the short-duration, high energy discharge typically provided by automotive batteries. Each standby storage battery includes one or more electrochemical cells, with multiple cells being connected in series so that the overall voltage, measured across the battery terminals, is equal to the sum of the individual cell voltages. Individual batteries are further connected together in series to form a battery bank having a higher voltage.
The potential difference, or voltage difference or differential, measured across the positive and negative terminals of a battery cell, is a characteristic of the particular chemistry of that cell. For example, in lead-acid batteries, the open circuit cell voltage is approximately 2.0 volts, while in a nickel-cadmium cell, the potential difference is approximately 1.2 volts. In each cell, positive and negative reactants, or active materials, are bound into a number of metallic grids to form positive and negative plates, respectively. Plates of like polarity are attached to a rigid, metallic supporting strap, which is fitted with a terminal post for connection to external loads. The assemblies of positive and negative plates with their respective straps and terminal posts are suspended in a jar or container containing a solution, which is an electrolyte, and the plates are separated by insulating material to ensure that no direct contact occurs between plates of opposite polarity. Such contact between the plates would result in a short circuit which would quickly discharge the cell, rendering it useless. The jar or other type of container is permanently sealed by means of a cover with holes therethrough. The positive and negative terminal posts protrude through these holes or openings.
When an electric load is connected between the positive and negative terminals of the battery, a spontaneous chemical reaction occurs between the electrolyte and the active materials in the battery plates and causes an electric current to be delivered into the load. The battery discharges by delivering DC current to the connected load. While the battery discharges, its terminal voltage gradually declines. The chemical reaction in the battery is reversible, i.e., nearly all of the energy removed from the battery during discharge can be returned by forcing a charging current of opposite polarity through the battery. The ability of a stationary storage battery or standby battery to deliver energy to a load, i.e., the battery's capacity, is defined by a non-linear relationship of battery voltage to the duration and magnitude of discharge current. This battery capacity is commonly expressed in ampere-hours (AH). For example, if the starting terminal voltage of a fully charged 100 AH lead-acid battery is 2.22 volts, and if the nominal discharge voltage is 1.75 volts, the battery can typically supply a current of approximately 12 amperes continuously for 8 hrs. before its terminal voltage reaches 1.75 volts. At 24 amperes, however, the voltage may decline to 1.75 volts in only two to three hours. Manufacturers generally develop and supply charts showing curves and tables relating current, time and voltage for their batteries. Other factors which may affect battery capacity include temperature, thickness of the active material in the plates, plate surface area in contact with the electrolyte, specific gravity of the electrolyte and the internal resistance of the cell.
A storage battery, like any source of electrical energy, has an internal impedance, which includes resistive, inductive and capacitive components. When the battery is discharging only direct current or DC is involved; capacitive and inductive components of the impedance have no effect on the power delivered by the battery. As the battery discharges, the current produces a voltage drop across the internal resistance of the battery in accordance with Ohm's law. This voltage drop causes the voltage across the battery terminals to be somewhat less than an "ideal", that is, the expected voltage, and the voltage drop consequently diminishes the ability of the battery to power the load. The internal resistance of a storage battery at the time of manufacture is made as low as possible to minimize the voltage dropped during battery discharge. A typical 200 AH lead-acid battery may have an initial internal resistance as low as 0.5 milliohms. Over the life of the battery, however, the internal resistance will increase, at a rate determined by such factors as how many times the battery undergoes cycles of discharging and recharging, the effects of continuous charging on the electrical conductivity of internal cell connections (e.g., electrochemical corrosion of the positive grid alloy), and the temperature of the electrolyte. The internal resistance of any cell will eventually increase to a level where the voltage drop across it during discharge is so great that the battery can no longer deliver power at its rated capacity. It is typical for batteries to be replaced when actual capacity falls to 80% of rated capacity, as determined by a controlled discharge test.
In most cases, internal cell resistance will not cause serious problems until the battery is near the end of its useful life, typically 12 to 15 years. In some cases, however, an increase in internal resistance of a cell will be abnormally rapid. Within only one or two years of its life, the cell resistance will become high enough such that heating effects from high discharge currents (so-called ohmic, or I.sup.2 R, heating) will melt or vaporize metallic connections among internal cell components, causing the battery circuit to open. An increase of only 500 milliohms in the internal resistance of a 200 AH lead-acid battery bank, for example, could result in such an open circuit under heavy load. A frequent cause of rapidly increasing cell resistance may be due to defects in the manufacturing process, including a faulty connection of internal cell elements, improper plate formation and certain deficiencies of the grid alloys.
If the battery circuit opens (an internal battery condition), it will no longer be able to discharge power into the load, and will consequently be useless as a standby energy source. Moreover, flammable gases released during charging reactions may accumulate within the battery jar or container, and may ignite when internal connections burn (by ohmic heating), causing an explosion that may damage equipment and seriously injure personnel. It is, therefore, very advantageous to have advance warning of an abnormal increase in cell resistance.
U.S. Pat. No. 4,968,943 to Russo discloses an open battery bank detector. Russo's non-intrusive open battery bank detector senses an alternating current component of the DC trickle charge carried by one of a pair of cables connected between the battery charger and the bank of batteries. When the AC component reaches a threshold level, a sensor circuit trips a relay which activates an alarm.
U.S. Pat. No. 4,546,309 to Kang discloses an apparatus and a method for locating ground faults. The Kang device utilizes a low frequency current generator having a variable output, a Hall-effect current probe for detecting the low frequency current produced by the generator, a filter and an amplifier connected to the output of the Hall-effect current probe for identifying and amplifying the low frequency signal. A readout element is connected to the output of the amplifier to indicate the relative magnitude of the low frequency signal.
U.S. Pat. No. 4,697,134 to Burkum discloses an apparatus and a method for measuring battery condition. The Burkum device measures the impedance of secondary cells that form the battery. The impedance measurement is made at a frequency selected to be different from those frequencies otherwise present in the charger-load circuit. A first application of the testing device monitors the battery for a change in impedance that can signal a developing defect in one or more individual cells or intercell connections. In a second application, the testing device is utilized to compare the impedance of individual cells and electrical connections to locate faulty components. A digital measurement of the measured AC current at the selected frequency is supplied to a computer or a digital system. A digital version of the measured voltage across the battery at the selected frequency is also supplied to the computer. The computer divides the voltage by the current measurement and records or logs the resulting impedance value on a regular basis.
U.S. Pat. No. 5,214,385 to Gabriel discloses an apparatus and a method for utilizing a polarization voltage to determine charge state of a battery. The test signal is a continuous square wave signal having a frequency less than 3 Hz. The test signal alternates between a voltage adequate to charge the battery and a lower voltage. The charging voltage is retained for a time sufficient to allow a polarization voltage to develop across individual battery cells.
U.S. Pat. No. 5,281,920 to Wurst discloses an on-line battery impedance measurement device. The impedance of battery cells within a battery bank is measured by dividing the bank into at least two battery strings. A load current is imposed on one of the battery strings and the battery cell voltage is measured within that string.
U.S. Pat. No. 5,574,355 to McShane discloses a method and an apparatus for the detection and control of thermal runaway in a battery under charge. The circuit determines the internal resistance or impedance and conductance (admittance) of the battery during the charge cycle. Since the charging current to the battery increases with time under a thermal runaway condition, a variable voltage source is connected between a regulator, which measures the voltage between the terminals of a battery and controls the voltage of the voltage source charging the battery, and the battery bank. The variable voltage source is connected between that regulator and the battery and introduces an offset voltage. By increasing the apparent voltage measured by the regulator between the battery bank terminals, the variable voltage source can be used to regulate the primary voltage source and thus reduce the charge applied to the battery upon detection of an impending thermal runaway condition. A microprocessor determines the conductance of the battery bank by applying a current pulse to the system with a current source. Conductance is the change in current flowing through the battery due to the current source divided by the change in the voltage of the battery due to the applied change in current. The microprocessor can also utilize impedance to detect a deteriorating condition. Thermal runaway is detected by the microprocessor when the absolute or relative conductance/impedance value exceeds a threshold, when the rate of change in the conductance/impedance exceeds a threshold, or a relationship between the ambient temperature (as measured by a temperature sensor) and the battery conductance/impedance exceeds a threshold. Upon detecting a thermal runaway condition an alarm is sounded.